In normal context, clicking refers to a noise made by a thumb and finger. On this note, to "click someone", means to make them generate a pleasurable moan in regards to sexually activity, usually handjobs.

"Damn he must have clicked her well!"
"You're gonnna get clicked..."
"I'll be clicking you later on tonight!"

Typically in any online video game, (usually an MMORPG or any other game where you have a lot of different abilities) where instead of keybinding (the act of making your abilities bound to a letter on the keyboard), you painstakingly look through each on of your moves and physically click the on them in order to preform them.

This is a terrible habit, and is almost always frowned upon by "pro " players, quite simply because clicking your abilities is at least 100x slower then moving your finger slightly to the right to press the R button in order to kill someone, especially when you have a million different buttons to press and the situation requires you to press them quickly.

Keybinding your abilities will improve how good you are at any PC video game by an unmeasurable amount.

A "Clicker" is a common derogatory term for someone who clicks their spells, or for someone who sucks so bad that they could possibly be clicking, although they are not. (rarely used as form two)

A player with their moves bound to their keyboard will have upwards of 100% faster reaction time then someone who is clicking their abilities.

Clicker: WTF!
Experienced Player: What?
Clicker: This guy keeps on owning me before I can even react! How is he so good?
Experienced Player: Well what are your keybindings like? Are they comfortable?
Clicker: Keybinds?
Experienced Player: Noob.